In wireless communication systems, electromagnetic radiation is transmitted from one or more antennas to communicate information. One characteristic of the electromagnetic radiation is its polarization. Polarization is a property that describes the orientation of the oscillation of the electromagnetic radiation.
Electromagnetic radiation has electric and magnetic fields that are perpendicular to each other and perpendicular to the direction of wave propagation. The electric field can be defined by a vector having X and Y components and traveling in the Z direction of a coordinate system. The polarization of the electromagnetic radiation is defined by specifying the orientation of the electric field vector at a point in space over a period of oscillation. If the X and Y components of the electric field have a sinusoidal oscillation with the same amplitude and are 90 degrees out-of-phase with each other, then the polarization is circular because the electric field vector traces out a circle in the X-Y plane. If the amplitude of the X and Y components are not the same, or if the phase difference varies from 90 degrees, then the polarization is defined as elliptical. In general, all polarizations can be considered elliptical. Circular and linear polarizations are special cases of elliptical polarization.
In some systems, the electromagnetic radiation is intended to be transmitted with circular polarization. Unfortunately, perfect circular polarization cannot be achieved in practical systems as there is always some, however small, polarization error. One measure of the quality of circular polarization is referred to as “axial ratio.”
Axial ratio can be calculated from the right hand and left hand circular components of the radiated electric fields as shown in equations (1)-(4) below. The left hand and right hand components are calculated from the complex X and Y components of the electric field as shown. Note: j=√(−1).
                              E          L                =                              E            x                    -                      jE            y                                              (        1        )                                          E          R                =                              E            x                    +                      jE            y                                              (        2        )                                AR        =                                                                                                        E                  R                                                            +                                                                E                  L                                                                                                                                      E                  R                                                            -                                                                E                  L                                                                                                                  (        3        )                                          AR          ⁡                      (            dB            )                          =                  20          ⁢                      log            10                    ⁢                                                                                                                        E                    R                                                                    +                                                                        E                    L                                                                                                                                                        E                    R                                                                    -                                                                        E                    L                                                                                                                                        (        4        )            
A channel with two communicating antennas having axial ratio greater than 0 dB will experience a polarization loss. Kales, M. L., “Techniques for Handling Elliptically Polarized Waves with Special Reference to Antennas: Part III-Elliptically Polarized Waves and Antennas”, Proceedings of the IRE, Volume: 39, Issue: 5: 1951, pp.: 544-549 shows in detail how to calculate the polarization loss factor (PLF) that must be applied in link budgets. As an example, two antennas with 4 dB axial ratio can have a maximum PLF of 0.9 dB. If the channel has one antenna with 1 dB AR and a second antenna with 3 dB AR, then the maximum PLF is 0.2 dB. It is desirable to minimize the PLF which can be done by minimizing each antenna's axial ratio. Therefore, there is a need in the art for improvements that reduce axial ratio in systems using circular polarization.